Anharmonic Lattice Vibrations in Small-Molecule Organic Semiconductors

TL;DR

This study reveals that lattice vibrations in small-molecule organic semiconductors transition from harmonic at low temperatures to anharmonic at higher temperatures, affecting their structural symmetry and electronic properties.

Contribution

It combines polarization-orientation Raman measurements with first-principles calculations to demonstrate anharmonic lattice dynamics and symmetry breaking in organic semiconductors.

Findings

Lattice vibrations are harmonic at 10 K.

Temperature increase causes loss of PO dependence in modes.

Evidence of a phase transition in pentacene between 80-150 K.

Abstract

The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, we use polarization-orientation (PO) Raman measurements to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, we show that at 10 K the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows a subtle phase transition between 80-150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. Our findings lay the groundwork for accurate predictions and new design rules for high-mobility organic semiconductors at room temperature.